The present invention relates to apparatus for measuring very small temperature differences, and more particularly a thermopile for measuring heat generated in a fluid sample while reducing thermal noise due to evaporation or thermal eddies in the sample or surrounding medium.
Such apparatus is used to measure temperature differences produced by an exothermic chemical reaction in close proximity to the thermopile, or by radiating the thermopile with infrared light. The apparatus is especially useful for determining analyte concentration by measuring the heat of reaction when a fluid sample is brought together with a reactive agent such as an enzyme, and provides an attractive alternative to colorimetric methods commonly used for this purpose.
A prior art thermopile of the type shown in FIG. 1 can be obtained as Model 2-142-1 from Barnes Engineering, Newark, CT. Such a thermopile can be used to measure infrared light. The use of a thermopile of similar design to measure the heat produced by a chemical reaction is described by E.J. Guilbeau et al in the ASAIO, Volume 10, No. 3, pages 329-335 (July-Sept. 1987).
Referring to FIG. 1, the prior art thermopile 13 comprises strips of antimony 1 and bismuth 2 which are electrically connected at measurement thermocouple junctions 3 and reference thermocouple junctions 4 but are otherwise separated by electrical insulation 5. Antimony and bismuth are well known for manufacturing thermocouples, and a cumulative voltage .DELTA.V is developed by the junctions 3, 4 and measured across contact pads 6' by a voltmeter.
In practice, the measurement junctions 3 are covered with a thermally responsive medium, and the reference junctions 4 are covered by a non-thermally responsive medium. For measurement of heat emitted by a body, the hot junctions (measurement thermocouples 3) are covered with black paint to absorb the heat. For measurement of the heat produced by a chemical reaction, the hot junctions are covered by a reagent layer in which a chemical reaction occurs. Such a reaction can be catalyzed by one or more enzymes, which results in an increased production of heat.
As the heat differences measured with these thermopiles are very small, the measurement is very sensitive to heat changes produced by thermal eddies (circulation of media, which has different temperature) or to temperature differences produced by evaporation of a liquid located on the thermopiles. The arrangement of thermopiles used by Muehlbauer et al. allows only the measurement of glucose in a fluid which has a certain linear velocity above the thermopile (M.J. Muehlbauer et al., Anal. Chem. 1989 Vol. 61, pages 77-83). In unstirred fluids a large temperature gradient can be measured between the reference and measurement junctions. A single drop of fluid (e.g. blood or glucose solution) over the thermopile results in a much higher thermal noise due to different evaporation, that means heat loss, over the junctions.
FIGS. 2 and 3 are schematic cross sections of the thermopile of FIG. 1. The antimony and bismuth elements are located on the bottom of a dielectric (electrically insulative) support 7, but for simplicity only the junctions 3, 4 are shown. The thermally responsive medium 8 is placed over the measurement junctions 3, and a drop of sample fluid 15 is applied. A thermally insulative medium 10 isolates the junctions from thermal noise outside the fluid.
FIG. 2 illustrates the case without containment, wherein the sample fluid 15 assumes a convex shape. FIG. 3 approaches this problem by extending the thermal insulation 10 to include sidewalls 11, but the fluid surface then assumes a concave shape. In either case the fluid undergoes a differential evaporation across its surface, which results in the thermal noise discussed above.
Reducing the size of the thermopile can reduce the thermal noise, but does not solve the noise problems attendant the prior art devices to a significant degree.